Infrared (IR) sensors rely on the inherent response of certain materials for capturing variations in the infrared component of an incoming light source. Infrared sensors are an essential component in a number of sensing applications in the industrial and research sectors, including, but not necessarily limited to, defense, quality control and inspection, surveillance, biomedical imaging, as well as research and development.
There are several drawbacks associated with the materials and technology used within modern infrared sensors, which result in degraded performance metrics, such as response time, sensitivity and others, as well as high costs. High resolution Mercury Cadmium Telluride (MCT) IR detectors, for instance, which rely on the photoconductive response of certain materials, are required to operate at temperatures approaching ˜70 K, preventing them from being widely used. Microbolometers, which can operate at room temperature, suffer from lower resolution (1) (i.e., Noise Equivalent Temperature Differential, NETD) as compared to photoconductive detectors, due to effects such as thermal cross-talk at room temperature.
Pyroelectric IR sensors that utilize IR-reflective top electrodes have lower sensitivity compared to both MCT and Microbolometers, due to the reflectivity of the top electrode, which is often patterned in order to maximize IR transmission to the active layer (2). The patterned, generally metallic, electrodes that are used in existing pyroelectric IR sensors either reflect or absorb IR, which limits the sensitivity, tunability and flexibility of the device. Patterning causes a trade-off between electrode coverage across the active area, which affects the magnitude of the output signal, and active area exposure, which affects the number of IR photons absorbed by the detector. Pyroelectric IR sensors that utilize Nichrome and other IR absorbing electrodes, limit the wavelengths that the detector can sense due to the well-defined absorption characteristics of the electrode. Furthermore, all IR sensors currently available are also restricted in terms of flexibility and thus must remain attached to static and planar monitoring equipment. The rigidity of current IR sensors leads to a requirement for complex optical components in order to correct for optical aberrations.
Commercially available IR sensors make use of either bolometric, photoconductive or pyroelectric properties of the active layer. These sensors typically comprise the active layer, an optional IR absorbing layer (in cases where the active layer has poor IR absorbance), a set of electrodes as well as a substrate. A brief discussion of the operation and drawbacks of existing technology follows.
Bolometric IR detectors, or microbolometers, make use of the temperature response of certain materials. That is, materials that exhibit a change in electrical resistance as a result of the temperature change due to the absorption of radiation. Microbolometers have two drawbacks; the first, is that in order to measure a change in resistance, a constant bias current needs to be passed through the device when it is in operation (2). Secondly, the accuracy of microbolometers is often degraded due to thermal crosstalk, which occurs due to thermal coupling between adjacent pixels. Thermal crosstalk occurs when the heat generated on one pixel due to impinging radiation, is conducted to an adjacent pixel through either the conducting electrodes or the substrate. In such cases, a resistance change observed at a given pixel may not be due to radiation impinging on that particular pixel, but rather due to heat transfer from an adjacent pixel leading to false detections.
Photoconductive IR sensors represent a class of high sensitivity sensors that are expensive and typically operate at low temperatures (˜70 K). Materials used within such sensors include Mercury Cadmium Telluride (MCT), Mercury Zinc Telluride (MZT) and others, many of which are difficult to grow in a controlled manner (3). Other limitations, such as low operating temperatures (in order to negate the effects of thermal noise), as well as the need for a constant bias current, prevent such devices from being used in applications other than military and research and development (4).
Pyroelectric sensors are a class of IR detectors that make use of the pyroelectric effect, which describes the spontaneous polarization/depolarization observed within the dipoles of certain materials as a result of a time-varying temperature change. Existing pyroelectric IR sensors consist of a top and bottom electrode, as well as an active pyroelectric layer. The top electrode typically consists of a patterned layer of conducting material such as gold or Indium Tin Oxide (ITO). Patterning is necessary due to the high infrared reflectivity of the metallic top contact, which causes much of the impinging infrared radiation to reflect away, rather than being absorbed by the IR absorbing layer (5). Having such a reflective top electrode is one of the primary drawbacks of existing pyroelectric sensors as it reduces the effective flux of incoming light and in turn lowers the sensitivity and responsivity of existing sensors. Some pyroelectric IR sensors may also use a top electrode consisting of an IR-absorbing material such as NiCr alloys (6). In these cases, the device is only responsive across the wavelengths of light that are absorbed by the NiCr, due to its predefined absorption spectrum.
There is therefore an ongoing need for improvements in infrared detectors. In addition, there is an ongoing need for improved detectors for other wavelengths of electromagnetic radiation.